I strongly agree with you that Splinterlands is not fun. The new game Soulkeep pales in comparison to Crazy Defense Heroes (which also has crypto elements.)
The only people who find Splinterlands fun are people who have not played any other games or hope to derive ROI. (Which is probably still easier to do in Splinterlands.)
Remove the monetary value, who is left?
And, yea, some people will find any game fun, but there are niche indie games too, and I doubt the majority will find those games fun.
I'm more willing to play Magic the Gathering Arena without earning money than play Splinterlands (after rewards tanked.) And you can argue that it's play to earn too! Because it's theoretically possible to win USD.
Splinterlands as it remains is far too expensive compared to other alternatives. For a tier 1 competitive deck, it's far cheaper in other games. (Sure those games don't have renting, but renting is a copout.)
For Splinterlands to become fun, that would require several fundamental changes, some of which greatly dilute the value of holdings that people have.
I'll throw in one more competitor of the same genre: Teamfight Tactics.
As long as wallet beats skill, I don't think Splinterlands can see mass adoption especially because the cost is so high. But if skill surpasses wallet, then that means, everyone who hasn't sold tanks to near zero.
If anyone wants to make a serious argument to play Splinterlands over any of the competitive games listed, I'm all ears. The argument must not hinge on the possibility of making money.